Capsaicin Application to the Temporomandibular Joint Alters Calcitonin Gene-Related Peptide Levels in the Trigeminal Ganglion of the Rat

The aim of this investigation was to determine the temporal effect of an intra-articular injection of capsaicin to the temporomandibular joint on the levels of calcitonin gene-related peptide-like immunoreactivity (CGRP-ir) in the trigeminal ganglion of the rat. The temporomandibular joints of 26 adult female rates were injected on one side with capsaicin and contralaterally with a control vehicle. Another 8 animals served as an untreated control group and received no injections. Animals were sacrificed at time intervals of 4 hours, 48 hours, 10 days, and 21 days following treatment. The trigeminal ganglia were extirpated, and CGRP-ir levels were quantified using a radioimmunoassay. Results demonstrated that when the capsaicin-treated side and teh vehicle-treated side were compared, CGRP-ir levels decreased initially at 4 hours and increased at 48 hours. At 10 days, CGRP-ir levels had again dropped below control lev els, followed by an increase at 21 days. CGRP-ir levels for the first two time periods investigated, which simulate an acute inflammatory state, mimic results observed in studies using limb joints, while the other time pe riods, which represent an intermediate and a chronic condition, respectively, suggest a more complex interaction with capsaicin-sensitive primary afferents.